1. Field of the Invention
This invention relates to a transmission apparatus, which utilizes a plastic fiber, for use in an optical communication system, and the like.
2. Description of the Related Art
Ordinarily, as light propagation paths in optical communication, single mode fibers or multi-mode fibers containing quartz glass as a principal material are utilized. The single mode fibers or the multi-mode fibers containing quartz glass as a principal material have a diameter of at most 200 μm. In alignment processes for the fibers, a high position matching accuracy on the order of micron is required. Therefore, fiber laying operations under ordinary environmental conditions, such as conditions at construction work sites, are not easy to perform. The difficulty of the fiber laying operations obstructs further popularization of the fibers described above.
Recently, plastic fibers, which have a comparatively large diameter and are comparatively easy to lay, have been proposed. However, for reasons of production processes, the plastic fibers primarily have a step index type of structure. The step index types of plastic fibers cannot transmit a signal of a high bit rate over a long distance. Specifically, in cases where a pulsed light signal as illustrated in FIG. 8A is inputted into a light entry end face of the step index type of fiber, a phenomenon is encountered in that a wave form of the pulsed light signal becomes deformed and spreads at a light radiating end face of the fiber as illustrated in FIG. 8B after being propagated over a long distance through the fiber. Therefore, in cases where a successively pulsed light signal as illustrated in FIG. 9A is inputted into the step index type of fiber and transmitted through the step index type of fiber, the problems occur in that, as illustrated in FIG. 9B, the pulses, which are adjacent to each other on the time axis, overlap one upon the other, and a perfectly extinct state cannot be obtained with respect to a logic “0” level of the signal at the light radiating end face of the fiber. In other words, in cases where the pulsed light signal of a short pulse width is transmitted through the fiber, a logic “0” level and a logic “1” level of the signal cannot easily be discriminated from each other after the signal has been transmitted through the fiber. Accordingly, the step index types of plastic fibers are not appropriate for large capacities of optical communication. The problems described above are described in, for example, “Fundamentals and Practice of Plastic Optical Fibers” by Yasuhiro Koike, supervised by Seizo Miyata, NTS K. K., pp. 84–87, 2000.
In order for the aforesaid problems to be eliminated, a graded index type of fiber, which has a comparatively large diameter and is free from an increase in pulse width of a pulsed light signal after being transmitted, has been proposed and is expected to be used in practice. However, it has been found that the problems described below are encountered with this type of fiber.
Specifically, the plastic fiber has the feature such that the diameter of the plastic fiber is capable of being set to be large. This means that, as illustrated in FIG. 7, in cases where the light signal having been propagated through the plastic fiber is to be detected and converted into an electric signal, it becomes necessary to utilize a photodetector 5, which has a light receiving section 4 having a large area corresponding to a cross-sectional area of a core 2 of a plastic fiber 1, in order for the light signal to be detected efficiently. (In FIG. 7, reference numeral 3 represents a cladding layer.) However, a light receiving device having a light receiving section with a large area corresponding to the core diameter of the plastic fiber, which core diameter is ordinarily at least 500 μm, has the problems in that the light receiving device has a large electric capacity and cannot always convert the quickly modulated light signal into the electric signal in response to the quickly modulated light signal.
More specifically, in cases where a successively pulsed light signal as illustrated in FIG. 10A is inputted into the plastic fiber, and the successively pulsed light signal having been propagated through the plastic fiber is detected by the light receiving device, which is provided with the light receiving section having a large area, at the light radiating end face of the plastic fiber, a detection signal is obtained such that, as indicated by a wave form A in FIG. 10B, the logic “0” level of the detection signal cannot be obtained with respect to the logic “0” level of the light signal, and therefore the logic “0” level and the logic “1” level of the light signal cannot easily be discriminated from each other.
In order for the aforesaid problems to be solved, it may be considered that a light receiving device, which has a small electric capacity and is capable of performing quick response, be utilized. However, the light receiving device, which has a small electric capacity and is capable of performing quick response, is capable of detecting only a small part of the light radiated out from the plastic fiber. Therefore, in such cases, a detection signal is obtained such that, as indicated by a wave form B in FIG. 10B, though the response characteristics are good, the intensity of the detection signal is markedly low.